I have this happen from time to time but I'm the only user. Are any of the client machines or the server machine using Time Machine? Or perhaps an off site backup utility that is allowed 100% bandwidth? Either of these can cause what you describe.

Update. Apple built us a duplicate same spec'd mini and we were able to repeat the problem dropouts. The only config that worked was the mini and two laptops connected to a router with ethernet - 3 foot cables. Hardly useful in our deployment as we have 8 users connecting from 600 mies away. We returned the 2nd mini and will be getting a MacPro instead.

I'd like to see your system log from the client machine (Tim), as this may let me know what caused the disconnect. Check your Inbox at the top of this page for instructions where to send the system log.